This invention relates to biological safety cabinets, for example, of the laminar airflow type and, more particularly, to a counterbalance system for the sliding window of a biological safety cabinet.
Biological safety cabinets are used in a wide range of laboratory, research, and other applications. Such cabinets include back and side walls defining a work area inside the cabinet, a horizontal work surface located between the back and side walls, an open front providing access by the operator to the work area, and an airflow and high efficiency filtration system to provide barrier protection at the front opening to prevent release of biological agents and airborne particulates to the surrounding environment.
The front opening of the cabinet may be provided with a vertically sliding window to selectively permit access to the work area of the cabinet in an open position, and to close off the interior of the cabinet in a closed position to provide personnel, product and environmental protection. Biological safety cabinets are typically manufactured in sizes such as 4, 5 and 6 foot wide with 2 foot high front openings. A sliding window made of glass is typically on the order of 1/4" (0.63 cm) thick and as such can weigh as much as 40 pounds. Prior art mechanisms to assist in raising and lowering sliding windows include weights and pulleys, extension springs and pulleys, and sash springs and pulleys. However, none of these prior art methods provides a substantially constant rate force spring, and thus at different window positions the user is required to apply substantially more or less force to operate the window.